Ubuntu – Reverse DNS is not a valid Hostname with existing PTR record matching HELO/EHLO

emailemail-serverpostfixsmtpUbuntu

There are a ton of questions related to this issue – and I have gone through almost each one yet this issue persists and I have no idea how to proceed further.

My mail server is setup with postfix, I have the corresponding A/AAAA records, etc setup including a PTR record from my IPv4 -> yourbud.co.za (my domain).

MXToolBox shows Reverse DNS does not match SMTP Banner. The reason being the HELO/EHLO response does not contain the domain.tld within the string. It does.

Requirements for SMTP server

  • Postfix setup (dkim, dmarc, spf all setup too)
  • A/AAAA records pointing yourbud.co.za -> ipv4/6 addresses
  • PTR record pointing ipv4 (and ipv6 optional?) -> yourbud.co.za

Postfix (postconf)

someuser@yourbud:~$ postconf -d smtpd_banner
smtpd_banner = $myhostname ESMTP $mail_name

someuser@yourbud:~$ postconf -d myhostname
myhostname = yourbud.co.za

MXToolBox

enter image description here

You can find the MXToolBox MX diagnostics for my domain here. Further, dmarc, dkim & spf all report passes.

Root Problem (I need to solve)

Outlook and GMail are marking my emails as junk.

More Information:

According to this, and this, this, and this (and a few others) all suggest that having a banner mismatch will cause mail providers (Outlook, GMail, etc) to mark the emails as spam/junk which is what is happening.

You can find more information about my question & my setup here.My IP is not on any blacklist site.

According to this post, MXToolBox requires a subdomain handling mail, something like mail.yourbud.co.za but this isn't a requirement as far as I can tell.

Why does Outlook/GMail still regard my mail as spam/junk?

Best Answer

Your setup is probably fine

A matching PTR is not a requirement by any standard, although some mail systems will use it as a potential spam indicator increasing the score. However, from the details provided in your question it's easy to come up with an answer that the warning shown in MXToolBox is indeed a false positive, and not the root cause for your messaged being marked as a spam:

  • Both IPv4 and IPv6 have matching forward and reverse records:

    yourbud.co.za. IN A 139.162.179.128
128.179.162.139.in-addr.arpa. IN PTR yourbud.co.za.

yourbud.co.za. IN AAAA 2a01:7e01::f03c:92ff:fed4:25b5
5.b.5.2.4.d.e.f.f.f.2.9.c.3.0.f.0.0.0.0.0.0.0.0.1.0.e.7.1.0.a.2.ip6.arpa. IN PTR yourbud.co.za.

  • SMTP banners on both IPv4 and IPv6 do match these records:

    Connection to 139.162.179.128 25 port [tcp/smtp] succeeded!
220 yourbud.co.za ESMTP Postfix (Ubuntu)

Connection to 2a01:7e01::f03c:92ff:fed4:25b5 25 port [tcp/smtp] succeeded!
220 yourbud.co.za ESMTP Postfix (Ubuntu)

How DNS, DKIM, SPF and DMARC are set up plays just a minor role as a positive indicator regarding spam filters. Failing the tests of course causes the message treated as forged, but that's all: because spammers can set up these too, they alone can't be used as a solid proof of the quality of the contents.

It's the contents that matters

From the web page https://yourbud.co.za/welcome#HowItWorks I can see you are working on an industry that can be easily misunderstood, and it's well possible that your messages are being marked as junk simply because of their contents:

How it works

  • Join South Africa's first Marijuana Co-Op Platform
  • Search the best marijuana in your area
  • Have it securely delivered to your door
  • Sit back, relax and enjoy your personal use marijuana legally

Susan Gunelius (Cannabiz Media): The Trouble with Email Marketing for Marijuana Businesses:

The reality is that marijuana is still illegal at the federal level, and that means most email marketing application providers don’t like marijuana-related businesses. Email marketing application providers require that their users follow “acceptable terms of use” and those terms typically prohibit sending messages about illegal drugs, goods, or services – including marijuana.

Microsoft, Gmail etc., being US companies, fit into this conclusion, too.

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